Uninterruptible power supply is an apparatus which supplies stable power to productive facilities at a momentary power interruption. More specifically, as shown in FIG. 1, the uninterruptible power supply (2) stores an extra electric energy from a power source (1) into an energy storage device (3) and supplies stable power to an output load (4) at a momentary power interruption.
Up to now, representative examples of the energy storage device of the uninterruptible power supply include a battery and an electric double layer capacitor. While the battery has a high energy density of 20-120 Wh/kg, it suffers from a low discharge density of 50-250 W/kg and a low cycle life of no more than 500 cycles. Although the uninterruptible power supply comprising the battery as an energy storage device has an advantage of elongated power supply due to the high energy density, it suffers from a disadvantage that regular maintenances caused by low life characteristics is required. To the contrary, the electric double layer capacitor has a high output density of 1000˜2000 W/kg and a semi-permanent cycle life, even though it has a low energy density of 1˜5 W/kg. For these reasons, the uninterruptible power supply comprising the electric double layer capacitor as an energy storage device is adequate for the backup of a power supply at the momentary power interruption.
The conventional uninterruptible power supply comprising the electric double layer capacitor as an energy storage device can be classified into two categories: an AC power backup system and a DC power backup system. The DC power backup system backups a DC electronic device installed within an output load to which the electric double layer capacitor is connected. Referring to FIG. 2, an AC input from an AC power source (1) is converted into DC power by the action of an AC/DC converter (5), and then an extra electric energy is stored into an electric double layer capacitor (3). At the temporary power interruption, the electric double layer capacitor (3) supplies the extra electric energy to the DC electronic device in a form of direct current in order to backup the device. Such a DC power backup system has a simple circuit constitution so that it is advantageous in terms of compactness and lightness. However, the DC power backup system can not applicable to the output load such as motors which requires alternative current as a power source. Further, the DC power backup system backups only the device positioned within the output load. Therefore, the system can not applicable to simultaneous backups of multiple output loads.
The AC power backup system supplies AC power to an output load. An AC input from an AC power source is converted into a DC power by the combinational action of a transformer and an AC/DC converter and then stored into the electric double layer capacitor. At a power interruption, the DC output from the electric double layer capacitor is converted into AC current by the action of a DC/AC inverter and then the AC power is supplied to the output load. This system supplies the same power with a power source such that it can backup multiple output loads at the same time. And the system can be applicable to the backup of the motors which requires AC power. Referring FIG. 3, an AC input from an AC power source (1) is converted into DC power by the combinational action of a transformer (8) and an AC/DC converter (5) and then, an extra electric energy is stored into an electric double layer capacitor (6). At the temporary power interruption, DC output from the electric double layer capacitor (6) is transferred into AC power by the action of a DC/AC inverter (9) and then supplied to an output load (4). Japanese unexamined patent publication 2001-061238 discloses an exemplary embodiment of the AC power backup system. Such an AC power backup system is a bulky and heavy system because of the transformer and the DC/AC inverter. Further, it has a low efficiency because of energy loss in the DC/AC inverter, which additionally increases the volume and weight of the system.